1. Field of the Invention
The present invention relates to a dispersion compensation apparatus for compensating for chromatic dispersion in an optical transmission line and, more particularly, to a dispersion compensation apparatus suitable for the transmission of wavelength-division multiplexed signals.
2. Description of the Prior Art
Development of higher-capacity optical communication systems is needed for the construction of future multimedia networks. So far, time-division multiplexing (TDM), optical time-division multiplexing (OTDM), wavelength-division multiplexing (WDM), etc. have been actively studied as multiplexing techniques having the potential of being able to achieve ultra high-capacity transmission. Among them, WDM transmission techniques hold great promise as a means for realizing a flexible lightwave network in which cross connects, branching and insertion, and multiplexing of different kinds of services are performed at the optical level by utilizing the wide gain region of erbium-doped optical-fiber amplifiers (EDFAs).
On the other hand, when performing optical transmission in the 1.5-.mu.m wavelength range by using 1.3-.mu.m single-mode fiber (SMF), it a general practice to cancel chromatic dispersion in a transmission line by using a dispersion-compensating fiber (DCF).
When using DCF in WDM transmission, if the operating wavelength range is made wide, a tilt (differences in loss) occurs along the DCF because of the wavelength dependence of DCF loss. This presents a major problem in transmission through multiple repeaters. For example, when transmitting 32 wavelengths with a channel spacing of 0.8 nm, a bandwidth of about 26 nm is needed, but it is difficult to even out losses in the DCF over such a wide band (for example, to within .+-.0.1 dB). Furthermore, when wavelength-division multiplexing further increases in capacity in the future, it is expected to use a bandwidth of 30 nm or wider, and in that case, making the DCF loss wavelength-independent will become a major challenge.
For example, when transmitting signals through SMF with 80-km linear amplifier repeater spacing, DCFs each with a dispersion value of around -1000 to -2000 ps/nm are needed. For the currently available DCF to have a total dispersion of -1000 to -2000 ps/nm will require a fiber length of about 10 km. If the WDM signal wavelength range is from 1530 nm to 1570 nm, since the DCF loss difference in that operating wavelength range is about 0.1 dB/km, a difference of about 1 dB will occur when a 10-km DCF is used. In transmission through several sections of linear amplifier repeaters, a total slope of 5 to 6 dB will occur in the DCFs. This will result in different S/Ns for different channels, and hence variations in transmission characteristics, thus causing a serious problem.